16. Kinetics II
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Define what is meant by the rate of reaction
The change in concentration/ amount of reactant or product produced per unit time
Give the techniques to measure rate of reaction
Electrical conductivity - - During a reaction there may be a change in the number of ions which will affect the level of conductivity. Measurement of change of mass Measurement of change in volume of a gas - gas syringe Titration - Small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent. Colorimetry - A common reaction is between propanone and iodine. The reaction goes from brown to colourless Measurement of optical activity The change in pH of a reaction
Give the rate equation
Rate (mol dm-3 s-1) = k[A]^m[B]^n K = rate constant which is fixed at a particular temperature. If the temperature changes so does the rate constant. - The larger the value of ‘k’ the faster the rate of reaction m and n = orders
Give the different orders of a rate reaction
0 order - The concentration of A has no effect on the rate of reaction 1st order - The rate of reaction is directly proportional to concentration of A 2nd order - The rate of reaction is proportional to the concentration of A squared
How do you confirm a reaction is second order
Plot a graph of rate against the concentration of R squared. The line produced should be a straight line through the origin. This shows rate is directly proportional to the concentration of [A]2
How do you work out half life and what is the half life like with first order, second order and zero order reactions
To work it out: Plot a concentration - time graph. Read the volume needed at zero. This represents the initial volume Find half of this value. Find this value on y axis and read off corresponding time. This is the first half life. Halve the volume you just read off from. Find this value on y axis and read off corresponding time. Subtract the first time value from this new time value.This is the second half life. Half life is always constant for first order (eg: time taken for the concentration of [A] to half is always 200 seconds). This means half life is useful for working out first order reactions without having to draw a rate concentration graph Half life increases for second order reactions. Half life decreases for zero order reactions
What is the iodine clock reaction used for and give the assumptions we are making
Iodine clock reaction can be used to work out initial rate of reaction. In clock reactions there are often two successive reactions (one slow and one fast reaction). An end point is achieved when one limited reactant runs out, resulting in a sudden colour change. Assumptions: The temperature remains constant Concentration of each reactant doesn’t change significantly over the time period
Describe the iodine clock reaction (13b)
Measure sodium peroxodisulfate, Na2S2O8, a few drops of starch solution (used as indicator to make end point more visible), and a known volume and concentration of potassium iodide into the beaker. Add known volume and concentration of sodium thiosulfate into beaker. Start the stopwatch Stop stopwatch when solution turns blue black Repeat experiment for many different concentrations. Keep the volume constant using varying amounts of distilled water to keep overall reaction volume the same. Always use the same volume of sodium thiosulfate because we Results Table with all the different runs and different concentrations of reactant used. From there you can work out orders of each reagent
In an iodine clock reactions, why is rate directly proportional to 1/time
In every trial the same amount of I2 forms before the blue black colour, so the amount of I2 is constant
We keep the total volume in the iodine clock experiment constant so concentration of [I-] is directly proportional to..
initial volume of I-
Why do we add a very small volume of sodium thiosulfate in the iodine clock reaction
Because we want to see initial rate Concentration of sodium thiosulfate does not affect rate of reaction
Give the equations for the iodine clock reaction
S2O82- (aq) + 2I- (aq) → I2 (aq) + 2SO42-(aq) 2S2O32- (aq) + I2 (aq) —> 2I- (aq) + S4O62- (aq)
What is the limiting reactant in the iodine clock experiment
Sodium thiosulphate - once that’s all used up, it wont react with iodine so the I2 stay in solution and react with starch to give a blue black colour
Describe how to work out initial rate using continuous monitoring (titrations) (13a)
Initial rate of reaction between iodine and propanone can be found by taking portions of the reaction mixture (containing vast excess of propanone, acid catalyst normally H2SO4, and servery limiting concentration of Iodine) at regular time intervals. You will need to quench the reaction in each sample by adding sodium hydrogen carbonate to remove acid catalyst/ significantly cool the reaction/ add another substance that reacts with and removes one of the reagents. Then titrate sample against sodium thiosulphate until solution in conical flask goes from brown to pale yellow. At this point add starch - it will go blue black if iodine (I2) is present. Keep adding sodium thiosulfate until it doesn’t go blue black. Repeat experiment several times changing concentration of iodine OR - Using a colorimeter to measure the concentration of iodine throughout the reaction. Notes: - Iodine decolourises from blue black to colorless during the reaction as it turns into iodopropanone and hydrogen iodide. - The concentration of iodine present in the portions will decrease as time increases - iodine is a reagent an is being used up Results: - The concentration (which is directly proportional to titre volume of sodium thiosulfate) time graph for iodine can then be drawn - o order so straight line sloping downwards Equations: Overall: CH3COCH3 + I2 (brown)+ H+ —> CH3COCH2I + HI (colourless) Titration: 2S2O32- + I2 —> 2I- + S4O62-
In the continuous monitoring method, titre volume of sodium thiosulfate is directly proportional to…
The concentration of iodine, I2
In a continuous monitoring method -eg: titrations, why must one substance be severely limited and the others be in vast excess
The severely limiting factor will affect rate as its concentration will change The severe excess reactants concentration won’t change much so only the limiting factor has an affect on rate
In continuous monitoring, when do you start the stopwatch
As soon as you add your last reagent - so this cant trickle down from something like a burette, it has to be poured in quickly from a beaker
Which reaction is always the rate determining step
The slowest reaction
How can you predict the rate determining step
You can predict the rate determining step from the rate equation - the reactant that appears in there must also appear in the rate determining step - if a reactant doesn’t appear in the rate equation, then it isn’t involved. The order of the reactant also tells you the number of molecules of that reactant in the RDS
Can catalysts appear in the rate determining step
Yes –> they are often H+ ions and are regenerated
Which halogenoalkanes use SN2 reaction pathway
Primary and secondary halogenoalkanes. There is 2 molecules involved in the rate determining step and rate equation: - Rate = k[haloalkane][nucleophile].
Which halogenoalkanes use SN1 reaction pathway
Secondary and tertiary An intermediate is formed with a planar shape around the carbocation, so the nucleophile could attack from either side There is only 1 molecule in RDS and rate equation: Rate = k[haloalkane]
When does the the nucleophile attack the halogenoalkane in an SN1 reaction
Nucleophile involved only after the rate determining step eg: Step 1: (CH3)3CBr -> (CH3)3C+ + Br- (slow) Step 2: (CH3)3C+ + OH- -> (CH3)3COH (fast)
What is formed with SN2
A transition state